1. Field of the Invention
This invention relates to a method of and a device for controlling a hydraulic press for compression molding a work.
2. Description of the Prior Art
Hydraulic presses are already known wherein a slide is moved by a hydraulic cylinder to compression mold a work. Such hydraulic presses are divided into various types depending upon a material and a shape of a work to be worked.
In order to compression mold a front panel, a body panel or a like part of an automobile using an SMC (Sheet Molding Compound) which is a thermosetting material in the form of a sheet, such a hydraulic press as disclosed, for example, in Japanese Patent Laid-Open No. 60-15119 is conventionally used.
The conventional hydraulic press disclosed includes a base, an upright mounted uprightly on the base, a crown provided at an upper portion of the upright, a hydraulic cylinder provided on the crown, and a slide supported at a lower end of a rod of the hydraulic cylinder for up and down movement under the guidance of the upright, whereby a lower metal mold half is secured to an upper face of the base while an upper metal mold half is secured to a lower face of the slide, and resin material is compression molded within a cavity defined between the upper and lower metal mold halves.
In compression molding an SMC material with the conventional hydraulic press, the hydraulic cylinder is controlled conventionally in such a manner as illustrated in FIG. 16. In particular, referring to FIG. 16, velocity control for controlling the velocity of the hydraulic cylinder at a plurality of stages is adopted for a period of time (t.sub.1, t.sub.2 and t.sub.3) until the slide is moved down from its top dead center to effect mold closing, and pressure control wherein the pressurizing force of the hydraulic cylinder is maintained fixed is adopted for another period of time (t.sub.4 and t.sub.5) until completion of compression molding after material has been filled into the cavity of the metal mold. Finally for a further period of time (t.sub.6 and t.sub.7) from completion of compression molding to mold opening, velocity control is adopted again.
In compression molding of resin material, it is important to effect pressure control of resin material in accordance with a variation in condition of the resin material.
The prior art, however, has a drawback that, because the pressurizing force of the press is fixed in pressure control, the press cannot follow a variation in condition of resin material and accordingly good molded articles cannot be produced.
In particular, the pressure of SMC material varies in such a manner as shown in FIG. 15. As seen in FIG. 15, the pressure rises suddenly at the instant when the metal mold half is contacted with the resin material so that the resin material is flowed in and fills up the cavity of the metal mold halves. After completion of such filling of the resin material, the SMC material is heated by an influence of the temperature of the metal mold halves and thus expanded in volume as seen in a zone denoted at a in FIG. 15. The SMC material then shrinks as seen in another zone denoted at b in FIG. 15, and finally it cures at a stage denoted at c in FIG. 15. A material which undergoes expansion, shrinkage and cure such as an SMC material thus varies in behavior and in pressure within a mold.
With conventional presses, however, pressure is constant and is not applied in accordance with a pressure of resin material. Accordingly, optimal pressure control for different conditions cannot be attained, and if pressure control is not appropriate, molded articles will have defects such as sinks and cracks and cannot be produced with a high quality.
Accordingly, in compression molding of a thermosetting resin material, appropriate transition from velocity control to pressure control and maintenance of an appropriate pressure in pressure control are very important.
Transition from velocity control to pressure control, however, is conventionally effected by changing over the velocity of movement of the slide at a plurality of stages in such a manner as shown in FIG. 17.
With the manner of changing over of the velocity, the velocity presents a discontinuous variation at each of such changing over points. Consequently, smooth deceleration of the slide cannot be attained and the metal mold half may be contacted too fast or too slowly with the resin material or may be contacted with the resin material while being vibrated. As a result, the resin material may not appropriately be filled up in the metal mold halves, which will have a bad influence on pressure control after then.
Further, depending upon a kind of resin material, it is necessary to move down the slide at a high velocity and then decelerate the slide rapidly and smoothly at a final stage of the downward movement. Such movement, however, cannot be attained with such a conventional multi-stage deceleration pattern as described above. Accordingly, it is a problem of the conventional presses that optimum molding cannot be assured for each resin material.
Changing over from velocity control to pressure control in the conventional hydraulic press described above is conventionally made by a setup of a timer.
Such a conventional change-over of control by a timer has following drawbacks.
In particular, in order to obtain excellent results with a pressure control within a range denoted at t.sub.5 in FIG. 16, it is necessary for resin material (SMC) to be completely filled in a metal mold. According to the change-over of control by a timer, however, control is changed over to pressure control only in response to an external factor (timer) whether resin material is filled up in a metal mold or not. Consequently, satisfactory results are not obtained by the conventional change-over of control by a timer.
For example, where the periods of time t.sub.3 and t.sub.4 shown in FIG. 16 are set too short, pressure control will be entered before material is filled up in a metal mold. Consequently, the resin material in the metal mold will not be acted upon by a pressure, and in an extreme case, this will give rise to such problems as an occurrence of a short shot or a crack.
On the contrary, where the periods of time t.sub.3 and t.sub.4 are set too long, resin material is filled up into a metal mold and begins to be heated and cure before a pressure is applied to the resin material. Thus, when a pressure is actually applied to the resin material, it will not act upon the entire resin material because the resin material has partially cured already. Consequently, a molded article having a sufficient strength cannot sometimes be obtained.
The change-over of control by a timer described above thus depends upon an assumption that resin material thrown into a metal mold is fully filled in the metal mold.
In recent years, further uniform molding is demanded with minimized burr, and supply of resin material is not fixed for each molding. Thus, it is not known clearly whether resin material is filled in a similar manner for each molding cycle, and consequently the assumption is not met satisfactorily.
In particular, the control by a timer described above is a one-sided controlling method which ignores the fact that filling conditions vary for each molding.
When, for example, flat plates are to be molded, resin material may be filled in different conditions in a metal mold or may be placed in different locations in a cavity of a metal mold. For example, resin material may be placed to a rather leftwardly displaced location within a cavity of a metal mold as shown in FIG. 18, or to a rather rightwardly displaced location as shown in FIG. 19. Besides, the amount of thus filled resin material may vary. Consequently, molded articles may vary in thickness or density, and only molded articles which are different from each other in a strict sense are obtained by different molding cycles.
As means for avoiding this to obtain molded articles as uniform as possible, a molding method is conventionally employed wherein an excessive amount of resin material is thrown into a metal mold so as to cause so-called burr to be produced on a molded article. The molding method, however, is inconsistent with the aforementioned requirement to minimize burr and thus deteriorates productivity significantly.
Also it is a problem of the prior art that, in pressure control, resin material is conventionally pressurized by a substantially fixed pressure as seen in FIG. 16 and consequently, good molded articles cannot be produced with a resin material which exhibits a variation in condition of expansion, shrinkage and cure.